Various types of piston filter systems have been used for filtering solid particles from liquid samples. Some of such filter systems are designed and intended as single use devices, i.e., they cannot be used for more than one single filtering of a single sample. Filter systems of this type are described, for example, in U.S. Pat. Nos. 3,512,940; 4,643,981; 4,800,020; 4,832,850; 4,897,193; and 4,990,253. Some of the devices are intended for quantitative separation wherein all of the liquid sample must pass through a filter.
There has not heretofore been provided a piston filter system in which the filter media can be replaced so that the filter system can be re-used. Usually the filter disc in the prior devices is held in place by a plastic retaining ring or is ultrasonically or heat sealed in place. This is reflected in the single-use application of such prior devices.